Inert gas enrichment in FCC unit regenerators

ABSTRACT

The concentration of sulfur trioxides in an FCC unit regenerator is maintained within environmentally accepted limits, while maintaining an adequate amount of gas for fluidizing conditions in the regenerator, by admixing the regenerator oxygen-containing gas with an inert gas. The quantity of the inert gas is controlled by a control loop measuring the pressure drop in the regenerator, and adjusting the amount of the inert gas to maintain the pressure drop within the predetermined limits.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of a copending application,Ser. No. 298,404, filed Sept. 1, 1981, now U.S. Pat. No. 4,395,325.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an improved method of operating a regenerationzone of catalytic cracking units.

2. Description of the Prior Art

Environmental limitations imposed by state and federal regulatoryagencies are becoming increasingly important considerations in theoperation of catalytic cracking units (e.g., fluid catalytic cracking--FCC units). In many areas of the country, and even in some foreigncountries, economic penalties, (e.g., reduced throughput, more expensiveraw materials) are being paid for the excessively high levels ofpollutants produced in the catalytic cracking operations.

A typical FCC unit comprises a reactor zone or vessel filled with acatalyst, and a regenerator vessel wherein spent catalyst isregenerated. Feed is introduced into the reactor vessel, and isconverted therein over the catalyst. Simultaneously, coke forms on thecatalyst and deactivates the same. The deactivated (spent) catalyst isremoved from the reactor zone and is conducted to the regenerator zone,wherein coke is burned off the catalyst with an oxygen-containing gas(e.g., air), thereby regenerating the catalyst. The regenerated catalystis then recycled to the reactor vessel. Some of the catalyst isfractionated into fines and lost during the process because of constantabrasion and friction thereof against the various parts of theapparatus. Most of the gaseous pollutants, formed in a catalyticcracking operation, are produced in the regenerator zone or vessel.

The efficiency of the regenerating operation is dependent on severaloperating parameters, the most important of which are regenerationtemperature and oxygen availability. In recent years most operators haveconcentrated on raising regenerator temperature to increase theefficiency of the regenerator zone through a complete or almost completecombustion of carbon monoxide in the regenerator vessel. This is mostcommonly accomplished by operating at air rates exceeding those requiredto burn the coke off the catalyst, and by introducing a carbon-monoxide(CO) combustion promoter, usually comprising at least one of thefollowing metals: platinum (Pt), palladium (Pd), rhodium (Rh), iridium(Ir), osmium (Os), and rhenium (Re). Some new regenerator designs haveincorporated better mixing methods for mixing coked catalyst with a COcombustion promoter and oxygen (e.g., fast fluidized bed regenerator ofGross et al, U.S. Pat. No. 4,118,338, the entire contents of which areincorporated herein by reference). However, while these new methods ofoperation of the regenerating vessel decrease the amount of carbonmonoxide exiting with the flue gas and improve the overall efficiency ofthe regeneration process, they may contribute to an increased level ofproduction of other pollutants, e.g., sulfur oxides, particularly sulfurtrioxide (SO₃), and nitrogen oxides (see for example Luckenbach, U.S.Pat. No. 4,235,704).

Simultaneously with the improved methods of operation of theregeneration zone, which alone may contribute to the increasedproduction of sulfur oxides in the flue gases of the regenerator, sulfurfeed levels in petroleum crudes available for cracking have beensteadily increasing over the past few years. In the past, due to overalllow levels of sulfur in FCC feeds, SO₃ levels in flue gases were low andgenerally only total SO_(x) levels were monitored without an SO₂ /SO₃breakdown, or without regard to the SO₃ levels. With the combination ofthe high sulfur feed levels, the high temperatures in the regenerationzone, and excessive air rates used in the regenerator, the SO₃concentration in the flue gas can be high enough to cause condensationin the flue gas which can result in a visible plume. The presence of avisible plume may violate local opacity requirements. In addition, theabsence of a plume indicates, in a vast majority of cases, that the SO₃emissions have not reached the maximum allowable limit.

The excessively high levels of SO_(x) and SO₃ are particularlyexperienced when it is necessary to reduce throughput, as required, forexample, by seasonal shifts in demand for FCC products, or shifts due toupstream or downstream processing problems. The SO₃ levels areparticularly high under those circumstances because the modernregenerator designs (e.g., those of U.S. Pat. No. 4,118,338) require gasvelocities in the combustor section sufficiently high to provideentrainment rates greater than the catalyst circulation rate. However,as mentioned above, the operation of the regenerator at such highoxygen-containing gas velocities, at reduced throughputs, results inexcessive levels of sulfur oxides (SO_(x)), and especially sulfurtrioxide (SO₃), in the flue gas. Such high levels of SO_(x) may violatecurrent environmental regulations.

SUMMARY OF THE INVENTION

An inert gas is admixed with the regeneration gas stream prior to theintroduction of the latter into the regenerator. The additional volumeof the inert gas maintains the catalyst fluidized to a desired degree atthe low throughput conditions. However, the formation of excessiveSO_(x) levels is prevented because the flow rate of oxygen-containingregeneration gas can be decreased without the loss of fluidizationconditions in the regenerator bed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of one exemplary embodiment of thepresent invention using externally-supplied inert gas to fluidize thecatalyst bed in the regenerator.

FIG. 2 is a schematic representation of an alternative exemplaryembodiment of the present invention using flue gas recycle as an inertgas to fluidize the catalyst bed in the regenerator.

DETAILED DESCRIPTION OF THE INVENTION

The inert gas is any gas which does not chemically react with, or haveany adverse physical effect on, the catalyst, the carbon monoxidecombustion promoter admixed with the catalyst, the feed, or the productsof the FCC process. Suitable inert gases are, for example, nitrogen,flue gas obtained from the FCC regenerator, helium or argon. If theinert gas is a flue gas obtained from the FCC regenerator, it can beadmixed with the oxygen-containing regeneration gas either before orafter it undergoes physical (e.g., cyclone), or any other (e.g.,electrostatic precipitator), cleaning treatment. The point from whichthe regenerator flue gas is taken to be recycled for admixture with theregeneration gas (i.e., before or after undergoing cleaning treatment)will depend to a large extent on individual unit economics and operatingconditions. Thus, the flue gas which has been passed through theregenerator cyclone system, but not through an electrostaticprecipitator, has a larger proportion of fines and a higher temperaturethan the flue gas which has undergone the electrostatic precipitatortreatment. Conversely, the flue gas which has been conducted throughboth, the cyclone system and the electrostatic precipitator system, hasa lower solids fines content, but it also has a lower temperature. Thus,an FCC operator, as will be obvious to those skilled in the art, has achoice, depending on the particular process conditions, of using eithera high temperature, relatively high solids content gas, or a lowertemperature, relatively low solids content gas, for admixture with theoxygen-containing gas introduced into the regenerator. In any event, therecycle of the flue gas into the regeneration vessel decreases thesulfur oxide emissions from the regenerator. Repeated contacting of thesulfur oxides (SO_(x)) with the catalyst in the regenerator increasesthe driving force for the SO_(x) capture onto the catalyst which, afterthe regeneration cycle is completed, is returned to the reactor portionof the FCC installation. In the FCC reactor, the entrapped SO_(x) isreleased as hydrogen sulfide (H₂ S) to a sulfur recovery section.Insofar as H₂ S is routinely recovered as sulfur in a conventionalsulfur recovery process, the release of the sulfur in the form ofhydrogen sulfide from the reactor vessel is preferred to increasedSO_(x) or SO₃ emissions from the regenerator vessel.

The amount of inert gas admixed with the oxygen-containing regenerationgas depends on the amount of gas that is necessary in the regeneratorfor maintaining the fluidized bed conditions, and, at the same time, onthe concentration of sulfur oxides in the flue gas exiting theregenerator. The catalyst bed in the regenerator is fluidized, so thatapparent catalyst density in the regenerator is about 10 to about 30pounds per cubic feet (lb/ft³), preferably about 12 to about 20 lb/ft³,and most preferably about 14 to 16 lb/ft³. The apparent density can bemeasured either directly by an appropriate density measuring instrument,or indirectly, e.g., by measuring pressure differential or pressure dropin the regenerator at an appropriate location. The data from the meansmeasuring the degree of fluidization (e.g., the density or the pressuresensor) may be relayed to an operator who manually adjusts the flow rateof an inert gas in a direction to minimize the deviation of the measuredparameter from the predetermined value thereof. Alternatively, thesensor measuring a parameter relating to the degree of fluidization maybe a part of a control loop comprising a controller regulating the flowrate of the inert gas into the regenerator. In response to themeasurement of a control parameter (e.g., ΔP or density) different fromthe preset value thereof, the controller compares the measured value tothe preset value to arrive at the control parameter deviation. Thecontroller then adjusts the flow rate of the inert gas in a direction tominimize the deviation. The control parameter may be set at any value,depending on the mode of operation of the regenerator. The appropriatelocation for measuring the catalyst bed density depends on the type ofthe regenerator used in the FCC installation, process conditions andoperator preference. It will be obvious to those skilled in the art thatany convenient location may be chosen for measuring the bed density, andthat the density of the regenerator bed can be maintained at anyconvenient level in accordance with this invention.

The SO₃ concentration can be monitored by any convenient means, e.g.,directly by an SO₃ analyzer or, indirectly, by an oxygen (O₂) analyzerin the flue gas line. Preferably, however, the SO₃ concentration iscontrolled indirectly by an oxygen analyzer in the flue gas line. Theanalyzer measures excess oxygen in the flue gas line. The amount ofexcess oxygen in the flue gas is maintained at a minimum (e.g., 0.0% to1.0% by mole) to substantially assure that the SO₃ concentration ismaintained at or below environmentally acceptable limits to assure thatthere is no visible plume in the regenerator flue gas. The data from theoxygen analyzer can be relayed to the process operator, who in turnmanually adjusts the amount of the regeneration gas conducted into theregenerator to maintain the oxygen level in the flue gas within thepredetermined limits. Alternatively, the analyzer may be a part of acontrol loop comprising, in addition to the analyzer, a controllerregulating the flow rate of the regeneration gas conducted into theregenerator. In response to an excessive oxygen concentration level inthe flue gas, the controller decreases the flow rate of the regenerationgas. Conversely, in response to a lower oxygen concentration level thanthat preset in the controller, the controller increases the flow rate ofthe regeneration gas.

The process of this invention can be used with any regenerator designwhich requires a substantial amount of gas for maintaining fluidizationconditions in the regenerator. However, the process is particularlyapplicable to the operation of riser type regenerators used in FCCunits, processing high sulfur feedstocks, e.g., those disclosed in Grosset al, U.S. Pat. No. 4,118,338, the entire contents of which areincorporated herein by reference.

The process of this invention can also be used with anyconventionally-used catalytic cracking feeds, such as naphthas, gasoils, vacuum gas oils, residual oils, light and heavy distillates andsynthetic oils. Suitable catalysts are conventionally used catalyticcracking catalysts, e.g., those containing silica and silica-alumina ormixtures thereof. Particularly useful are higher and lower activityzeolites, preferably low coke-producing cyrstalline zeolite crackingcatalysts comprising faujasite crystalline zeolite and other zeolitesknown in the art. The carbon monoxide burning promoter optionally usedin the process is any conventionally used carbon monoxide burningpromoter, such as platinum (Pt), palladium (Pd), rhodium (Rh), iridium(Ir), osmium (Os), or rhenium (Re). The amount of the carbon monoxideburning promoter in the bed of catalyst is maintained at a conventionallevel, as disclosed e.g., by Schwartz in U.S. Pat. No. 4,072,600. Theregenerator procedure for the catalyst containing the promoter ispreferably that particuarly promoting the recovery of available heatgenerated by the burning of carbonaceous deposits produced inhydrocarbon conversion, such as that disclosed in U.S. Pat. No.3,748,241 and 3,886,060, the entire contents of both of which areincorporated herein by reference.

The invention will now be described in conjunction with two exemplaryembodiments thereof illustrated in the FIGURES.

Referring to FIG. 1, a gas oil feed is introduced through a conduit 2into a riser reactor 4 along with a regenerated catalyst conducted tothe reactor from the regenerator 6 by a conduit 5. The feed volatilizesalmost instantaneously, and it forms a suspension with the catalystwhich proceeds upwardly in the riser. The suspension then passes into agenerally wider section of the reactor which contains solid-vaporseparation means 9, such as conventional cyclones. The catalyst isseparated from the products of the reaction, and is then conducted to astripping section 13, wherein entrained gases are removed from thecatalyst by steam.

Stripped catalyst containing carbonaceous deposits (i.e., coke) iswithdrawn from the bottom of the stripping section through a conduit 7and conducted to a regeneration zone or vessel 6. In the regenerationzone, the catalyst undergoes preliminary regeneration in a relativelynarrow combustion zone 11 by passing oxygen-containing gas, such as air,into the combustion zone and burning the coke off the catalyst. Thecatalyst suspension then proceeds into a relatively wider section 17 ofthe regenerator, wherein residual carbon and CO are combusted, and,finally, into a solids-gas separation section 21 containing separationmeans, e.g., cyclones, 19. In the separation section 21, the catalyst isseparated from the flue gas. The flue gas is then conducted to anoptional power recovery section 8, then to a cooler 10, and finally toan electrostatic precipitator 12 before it is discharged into theatmosphere. Inert gas, e.g, nitrogen, can be optionally supplied to theregenerator by a conduit 18 equipped with a valve 20. The inert gas maybe admixed with the oxygen-containing regeneration gas, e.g., air, by aconduit 24, before the regeneration gas is introduced into a blower 26,or through a conduit 22 after the regeneration gas is discharged fromthe blower 26. The amount of the inert gas admixed with the air isregulated by a control loop comprising a pressure sensor 25, acontroller 27 and a valve 20. The pressure sensor 25 measures thepressure drop in the combustor section 11, and relays this informationto the controller 27, equipped with a setpoint 29. The controller 27also controls the operation and the degree of opening of the valve 20.The pressure sensor 25 measures the pressure drop in the combustorsection across a distance of 62.4 inches. The measured value of thepressure differential across that distance corresponds directly toapparent density of the catalyst bed in the combustor section. Thus,optimum pressure differential across the 62.4 inches vertical distancein the combustor section is 14-16 inches of water, corresponding to anapparent catalyst density of 14-16 pounds per cubic foot (lb/ft³). Ifthe pressure differential detected by the sensor 25, and thus theapparent catalyst density, exceeds tht level, thereby indicating anexcessive amount of inert gas introduced into the regenerator,controller 27 decreases the opening of the valve 20 to decrease thedeviation between the optimum pressure differential value and themeasured value. Conversely, if the measured pressure differential valueis lower than the optimum value, controller 27 increases the opening ofthe valve 20 to introduce more inert gas into the regenerator tomaintain the fluidized catalyst bed at the desired density. Accordingly,the deviation (defined as the difference between the measured pressuredifferential value and the preset value) is decreased.

The amount of the oxygen-containing regeneration gas (air in thisexample) is controlled by a controller 16, having a set point 14. Theamount of the air introduced into the regenerator is controlled by valve31, which is controlled by the controller 16 in response to the oxygenconcentration in the flue gas monitored by an oxygen sensor 17. Theoptimum amount of oxygen (O₂) concentration in the flue gas is set inthe setpoint 14 at about about 0 to about 1 mole percent, preferablyabout 0 to about 0.7 mole percent, and most preferably about 0 to about0.5 mole percent. Control of the flue gas oxygen content within theaforementioned limits enables operator of the process to keep the SO₃emissions at such a level that the molar ratio of SO₃ /SO_(x) is lessthan 5 percent. If the concentration of O₂ detected by the detector 17in the flue gas exceeds the level set at the setpoint 14, therebyindicating an excessive amount of oxygen being introduced into theregeneration zone, controller 16 decreases the opening of the valve 20to decrease the intake of air into the regenerator. Accordingly, theconcentration of oxygen in the regenerator will decrease, and so willthe concentration of SO₃ in the flue gas. Conversely, if theconcentration of oxygen detected by the detector 17 is less than thelimit set in the setpoint 14, the controller 16 sends a signal to valve20 to increase the opening thereof, thereby increasing the intake of airinto the regenerator. Valve 20 may also be commonly controlled byoperator intervention to control the rate of air flow, and thus the SO₃content of the flue gas. However, it is preferred to operate the valve20 by means of an automatic control loop described above.

In an alternative embodiment, illustrated in FIG. 2, the inert gasadmixed with air (or any other oxygen-containing gas) is recycled fluegas. The flue gas may be recycled from three different alternativepositions shown in the drawing and discussed in detail below.

The operation of the reactor and the regenerator section of theapparatus of the embodiment of FIG. 2 is identical to that of theembodiment shown in FIG. 1, discussed above. The respective parts of theapparatus of FIG. 2 are numbered identically to those of the embodimentof FIG. 1, with a prefix of 100. Thus, for example, the gas oil feedline 102 of FIG. 2 corresponds to the gas oil feed line 2, and the riserreactor 104 of FIG. 2 corresponds to that of the riser reactor 4 ofFIG. 1. Accordingly, it is believed that the operation of the respectiveparts of the embodiment of FIG. 2 will be obvious to those skilled inthe art from the above description of the embodiment of FIG. 1. Flue gasis recycled in this embodiment from the exit of the regenerator vesselto the conduit 123 carrying oxygen-containing regeneration gas. The fluegas may be recycled into the conduit 123 from one of three differentlocations shown in FIG. 2. Thus, the flue gas may be recycled from theoutlet of the regenerator cyclone separation system via line 128 (shownas a phantom line in FIG. 2) to the suction side of the combustion airblower 126. If the flue gas is recycled directly from the outlet of thecyclone separation system, it provides a high pressure, relatively hightemperature gas with a relatively high catalyst fines content.

Alternatively, the recycle flue gas may be recycled from the inlet ofthe electrostatic precipitator through a conduit 129 (also shown as aphantom line in FIG. 2) to the intake side of blower 126. In thisembodiment, the recycled gas has lower temperature than that recycledthrough a conduit 128, because it passed through a power recoverysection 108 and a cooler 110.

In yet another embodiment, the recycled flue gas may be recycled fromthe outlet of the electrostatic precipitator 112 through a conduit 130to the blower 126. In this case, the recycled flue gas also has arelatively low temperature and a relatively low solids catalyst finescontent because of its passage through the electrostatic precipitator.The proper source for the recycled flue gas may be chosen by anindividual operator based on individual unit economics. However, asmentioned above, the recycle of the flue gas from any of the threeaforementioned points in the process aids in the reduction of theemission of oxides of sulfur (SO_(x)) from the regenerator. Repeatedcontact of the SO_(x) from the flue gas with the catalyst increases thedriving force for the SO_(x) capture onto the catalyst which,eventually, is returned to the riser of the reactor, wherein the SO_(x)is released as hydrogen sulfide.

An optional blower 132 may be placed in the flue recycle line 122 inorder to increase, if needed, the capacity of the main blower 126. Itwill be obvious to those skilled in the art that the control of theamount of flue gas admixed with the oxygen-containing regeneration gasis accomplished in the same manner as in the embodiment of FIG. 1 by acontrol loop comprising a controller 127 with a setpoint 129, a pressuresensor 125 and a valve 120 controlled by the controller 127.

It will also be obvious to those skilled in the art that the catalyticcracking process and apparatus of this invention may be conventionallyequipped with a number of other control loops normally used in catalyticcracking installations, and the operation of these conventional loopscan be integrated and/or can be kept independent of the operation of thecontrol loops discussed above. Such conventionally used control loops,and other details of FCC processes, are fully disclosed in the followingpatents and publications: U.S. Pat. Nos. 2,383,636 (Wurth); 2,689,210(Leffer); 3,338,821 (Moyer et al); 3,812,029 (Snyder, Jr.); 4,093,537(Gross et al); 4,118,338 (Gross et al); Venuto et al, Fluid CatalyticCracking with Zeolite Catalysts, Marcel Dekher Inc. (1979). The entirecontents of all of the above patents and publications are incorporatedherein by reference.

It will be apparent to those skilled in the art that the above generaldescription of the apparatus, the process and of the specificembodiments thereof can be successfully repeated with apparatus andingredients equivalent to those generically or specifically set forthabove and under variable process conditions.

From the foregoing specification, one skilled in the art can readilyascertain the essential features of this invention and without departingfrom the spirit and scope thereof can adopt it to various diverseapplications.

We claim:
 1. In a catalytic cracking process comprising:contacting ahydrocarbonaceous feed with a cracking catalyst to produce crackedhydrocarbon vapors and deactivated catalyst containing carbonaceousdeposits; separating the deactivated catalyst from the hydrocarbonvapors and conducting the deactivated catalyst to a regeneration vessel;regenerating the deactivated catalyst under fluidized bed conditions inthe regeneration vessel by means of an oxygen-containing gas introducedinto the regeneration vessel, thereby forming a flue gas comprisingoxygen, sulfur dioxide, sulfur trioxide, carbon monoxide and carbondioxide; the improvement wherein the oxygen-containing gas is admixedwith a stream of an inert gas, prior to the introduction of theoxygen-containing gas into the regeneration vessel, in the amountsufficient to maintain apparent catalyst bed density in the regenerationvessel at about 10 to about 30 lbs/ft³ and the amount of the oxygencontent in the flue gas at about 0 to about 1 mole percent.
 2. A processaccording to claim 1 wherein the inert gas comprises the regenerationvessel flue gas.
 3. A process according to claim 2 wherein theregeneration vessel flue gas is conducted through a regeneration vesselcyclone separation system before being admixed with theoxygen-containing gas.
 4. A process according to claim 3 wherein theregeneration vessel flue gas, after passing through the regenerationvessel cyclone separation system, is conducted through an electrostaticprecipitator.
 5. A process according to claim 1 wherein the inert gas isnitrogen.
 6. A process according to claims 1, 2, 3, 4 or 5 furthercomprising a method for controlling the oxygen content of said flue gas,which comprisescomparing the oxygen content of the flue gas with apredetermined value of the oxygen content to obtain an oxygen contentdeviation, and controlling the amount of the oxygen-containing gas in adirection to reduce the oxygen content deviation.
 7. A process accordingto claim 6 further comprising a method for controlling apparent densityof the fluidized catalyst bed in the regeneration vessel, whichcomprisescomparing pressure drop in the fluidized catalyst bed in theregeneration vessel with a predetermined value of the pressure drop toobtain the pressure drop deviation, and controlling the amount of theinert gas admixed with the oxygen-containing gas in a direction toreduce the pressure drop deviation.
 8. A process according to claim 7wherein apparent catalyst bed density in the regeneration vessel isabout 12 to about 20 lbs/ft³.
 9. A process according to claim 8 whereinapparent catalyst bed density in the regeneration vessel is about 14 toabout 16 lbs/ft³.
 10. A process according to claim 6 wherein thepredetermined value of the oxygen content of the flue gas is about 0 toabout 0.7 mole percent.
 11. A process according to claim 10 wherein thepredetermined value of the oxygen content of the flue gas is about 0 toabout 0.5 mole percent.
 12. A process according to claim 11 wherein theSO₃ emissions in the flue gas are maintained at such a level that themolar ratio of SO₃ /SO_(x) is less than 5 percent.